added session serializer

@manjunath24 :  We should avoid doing this, why will the JSON serializer not work? See the Django docs for more details. 

This attack can be mitigated by serializing session data using JSON rather than pickle. To facilitate this, Django 1.5.3 introduced a new setting, SESSION_SERIALIZER, to customize the session serialization format. For backwards compatibility, this setting defaults to using django.contrib.sessions.serializers.PickleSerializer in Django 1.5.x, but, for security hardening, defaults to django.contrib.sessions.serializers.JSONSerializer in Django 1.6. Even with the caveats described in Write Your Own Serializer, we highly recommend sticking with JSON serialization especially if you are using the cookie backend.